See this StackOverflow question for serving static files through expressjs: static files with express.js
You should be able to use Heroku's free dyno to handle a good bit of traffic since you're only serving static files.
I would suggest putting all of those files/folder in a "public/" directory and using this code:
var express = require('express');
var app = express();
app.use(express.static(__dirname + '/public'));
app.listen(process.env.PORT || 3000);